Air travelers have long sought the convenience and efficiency of widespread supersonic commercial aviation only to be denied by technological, economic, and political roadblocks. With operations spanning over a quarter of a century, the Concorde remains the only commercial aircraft that travels at supersonic speeds but struggles with technological obsolescence. Fuel consumption and maintenance requirements of the Concorde strain commercial feasibility in today's competitive environment. Possibly overshadowing other technological and economic shortcomings is the Concorde's thunderous sonic boom that is capable of shattering windows in buildings under the flight path, a burden that restricts the Concorde to routes over oceans.
The sonic boom creates a major practical risk of commercial supersonic aviation so long as commercial supersonic aircraft are prohibited from flying over populated land masses.
A sonic boom occurs due to pressure waves that form when an aircraft moves at supersonic speeds. During subsonic flight, air displaced by an aircraft flows around the configuration the same way water goes around an object in a stream. However, as the aircraft approaches supersonic speeds, air at the leading edge of the configuration is compressed to a non-linear threshold where discontinuities in flow properties, manifest through a pressure pulse, are generated as shocks and propagated through the atmosphere. Pressure pulse intensity decreases as a consequence of propagation through the atmosphere and changes shape into an N-shaped wave within which pressure rises sharply, gradually declines, then rapidly returns to ambient atmospheric pressure. A wall of compressed air that moves at aircraft speed spreads from the wave and, in passing over ground is heard and felt as a sonic boom. Rapid changes in pressure at the beginning and end of the N-wave produce the signature double bang of the sonic boom.
Research has recently shown that boom intensity can be reduced by altering aircraft shape, size, and weight. For example, small airplanes create a smaller amplitude boom due to a lower amount of air displacement. Similarly, a lighter aircraft produces a smaller boom since the aircraft rests on a smaller column of compressed air and the lighter plane generates a lower pressure column for a similar signature, for example N-wave. An aircraft that is long in proportion to weight spreads the overpressure across a greater distance, resulting in a lower peak pressure. Furthermore, wings that are spread along the body and not concentrated in the center as in a conventional aircraft have a greater lifting length and produce a pressure pulse that is similarly spread, resulting in a smaller sonic boom.
One technique for boom reduction is shaping. Shaped sonic boom refers to a technique of altering source pressure disturbance such that a non-N-wave shape is imposed on the ground. Shaping sonic boom can reduce loudness by 15-20 dB or higher with no added energy beyond that to sustain flight. Shaping to minimize loudness is based on insight regarding changes in aircraft pressure disturbances during propagation to the ground. During the sixties and seventies, Jones, Seebass, George, and Darden developed a practical analytical guideline for low boom design.
Studies have shown that sonic boom loudness at audible frequencies correlates with annoyance. Therefore supersonic over-land flight could only be achieved by reducing the sonic boom to acceptable sound levels. Shaped sonic booms are only achieved deliberately. No existing aircraft creates a shaped sonic boom that persists for more than a fraction of the distance to the ground while flying at an efficient cruise altitude, since non-shaped pressure distributions quickly coalesce into the fundamental N-wave shape. Audible frequencies for a sonic boom occur essentially at the beginning and end of a typical N-waveform. Shocks become quieter at decreasing magnitudes and increasing rise times of the pressure change. The N-wave form generates the largest possible shock magnitude from a particular disturbance. The N-wave shape results because the front of a supersonic aircraft generates an increase in ambient pressure while the rear generates a decrease in pressure. Variation in propagation speed stretches the disturbance during propagation to the ground. The disturbances stretch and also coalesce because shocks travel at speeds that monotonically change with magnitudes of the local pressure. Shaped boom techniques typically attempt to prevent coalescing of the pressure disturbance by adding a large compression at the aircraft nose and an expansion at the tail with pressure between constrained between compression and expansion. The shaped boom stretches the ends of the signature faster than the in-between pressures, creating a non-N-wave sonic boom at the ground.